MRAM is a non-volatile memory technology that is gaining popularity in the computer market. Unlike other memory technologies (e.g., SRAM, DRAM, FLASH, etc.) that store data as electric charge or current flows, MRAM stores data as a magnetic state in a magnetic storage element (i.e., an MRAM cell). Typically, an MRAM cell includes two ferromagnetic layers, each of which can hold a magnetic field that has one of two possible polarities. Preferably, the possible polarities of the ferromagnetic layers will run either parallel or anti-parallel to an easy axis of the MRAM cell. The logic state of the MRAM cell may then depend on the polarity of the ferromagnetic layers. For example, if the ferromagnetic layers have the same polarity, the MRAM cell may be storing a “0.” Alternatively, if the ferromagnetic layers have an opposite polarity, the MRAM cell may be storing a “1.”
Generally, a device may read the stored data in an MRAM cell by passing a sensing current through the MRAM cell and then measuring an electrical resistance of the cell, which relates to the polarity of the ferromagnetic layers. In this respect, a higher resistance Rmax typically indicates that the ferromagnetic layers have an opposite polarity, whereas a lower resistance Rmin typically indicates that the ferromagnetic layers have the same polarity. A device may write data to an MRAM cell by applying to the MRAM cell a magnetic field that alters the magnetic state of one or both of the ferromagnetic layers. In this respect, the device may pass current through write lines adjacent to the MRAM cell to generate the magnetic field.
One example of an MRAM cell is a pseudo spin valve (PSV). The PSV may include two magnetic layers of different thicknesses, separated by a nonmagnetic conductive spacer layer. Both magnetic layers are “free” layers, meaning that both layers can switch polarity when subjected to an applied magnetic field. However, the thicker layer of the PSV, which stores the data, may require a larger magnetic field to switch its polarity (i.e., the thicker layer has a higher “switching field”). When reading a PSV, a device may measure the resistance of the PSV while applying magnetic fields that magnetize the thinner layer of the PSV, but not the thicker layer, in known directions. In this respect, the device may compare the resistance of the PSV when the thinner layer is magnetized in one direction with the resistance of the PSV when the thinner layer is magnetized in an opposite direction to determine the polarity of the thicker layer, and thus the logic state of the PSV. Typically, the difference in resistance for the two logic states of the PSV is approximately 5% (i.e., Rmax is approximately 5% higher than Rmin).
Another example of an MRAM cell is a spin valve (SV). The SV may also include two magnetic layers separated by a nonmagnetic conductive spacer layer (e.g., Cu), and one of the SV's magnetic layers may be a free layer that stores data. However, the other magnetic layer of the SV may be a “pinned” layer, meaning that the layer's polarity is fixed in a known direction by an anti-ferromagnetic layer. As such, when reading an SV, a device may measure a resistance of the SV and then determine the polarity of the free layer, and thus the logic state of the SV, based on that resistance and the known direction of the fixed layer. For example, a device may compare that resistance to the resistance of a reference element. Similar to a PSV, the difference in resistance for the two logic states of the SV may be approximately 5%.
Yet another example of an MRAM cell is a magnetic tunnel junction (MTJ). Similar to the SV, the MTJ may include a free magnetic layer that stores data and a pinned magnetic layer. However, in the MTJ, the magnetic layers are separated by a nonmagnetic insulating barrier layer, as opposed to a conductive spacer layer. In this respect, a tunneling current may flow perpendicularly between the free layer and the pinned layer through the barrier layer. When reading an MTJ, a device may measure the resistance of the MTJ and then determine the polarity of the free layer, and thus the logic state of the MTJ, based on that resistance and the known direction of the fixed layer. For example, a device may compare that resistance to the resistance of a reference element. Unlike the PSV and SV described above, the difference in resistance for the two logic states of the SV may be approximately 50%. As such, read errors may be less likely in MRAM with MTJ elements.
The MRAM cells described above may have various limitations. For example, the PSV may require a destructive read process that includes writing two opposite bits to the thinner layer of the PSV. As such, reading the PSV may require additional power and time during that destructive write process, and may also create the potential for write errors. As another example, the PSV and the SV described above may only provide a resistance difference of 5% between logic states, which may lead to read errors. As a further example, the SV and MTJ described above may require a read process that relies on a reference element, which may impact the reliability of the read process and thus lead to read errors. As still a further example, the MRAM cells described above may provide a low output signal, thus making it difficult for a read architecture to accurately measure the signal and/or determine a logic state of the MRAM cell based on that signal. As still another example, the magnetic storage elements described above may only store a single bit of data. As such, there is a need for an MRAM cell that overcomes one or more of these limitations.